Laser printers and related optical imaging systems often employ a raster-scanned optical beam or beams produced by an optical source to render images (e.g., a printed image). For example, early laser printers typically employed a single raster scanned optical beam, often generated by a laser or similar optical emitter. To render an image, the single optical beam was focused to form an illumination spot on a photoconductive surface. The optical beam was modulated to modulate the illumination spot as it was scanned across the photoconductive surface. The scanned illumination spot exposed a latent image along a scan line on the photoconductive surface yielding a pattern of relatively charged and uncharged surface regions along a length of the scan line. Using some form of toner or ink (e.g., solid or liquid) that differentially adheres to the charged and uncharged regions, the latent image was developed. The toner, patterned according to the latent image, was then transferred to paper or a similar substrate to render the printed image.
Over time, an interest in greater printing speed and increased overall throughput, has given rise to the use of multi-beam laser scanning units (LSUs) that provide more than one optical beams. In printers that employ multi-beam LSUs, each of the multiple beams produces an independently modulated illumination spot. In turn, each separate modulated illumination spot is used to expose a corresponding separate scan line on the photoconductive surface. Together the modulated illumination spots and resulting separate scan lines enable printed images to be produced at a much faster rate than is generally possible with single beam LSUs. However, while adding optical beams enables faster printing and a concomitant increase in throughput of a printer, using multiple optical beams does present some challenges not present in the single beam LSU. For example, along with the use of multiple optical beams comes the problem of producing illumination spots having a desired or targeted spot size while simultaneously maintaining a desired or targeted separation or spacing between the scan lines or equivalently between the illumination spots at the photoconductive surface.
One approach to providing simultaneous, substantially independent, control of both an illumination spot size and an effective illumination spot spacing (or more properly control of scan line spacing) is to tilt a linear arrangement or pattern of the illumination spots relative to a scan direction of the photoconductive surface. In particular, the tilt may be used to establish a target spacing between the scan lines while optics of the LSU is employed to separately determine the target spot size of the illumination spots on the photoconductive surface. A tilt angle of about 86.4 degrees, for example, may be used to produce a tilted linear pattern of illumination spots that is oriented almost parallel to an optical scan direction at the scanning surface to provide a target scan line spacing of about 0.03125 millimeter (mm) or about 32 lines/mm for an actual illumination spot spacing of about 0.5 mm. Further, the scan line spacing may be adjusted by a substantially arbitrary amount through fine-tuning of the tilt angle.
Unfortunately, while tilting the linear pattern of illumination spots may enable independent control of spot size and spacing of the illumination spots, linear pattern tilting tends to introduce other issues that effectively limit a practical number of optical beams that may be employed. In particular, the number of optical beams as well as a maximum achievable scan width (i.e., scan line length) may be severely limited due to focal plane separation, wherein each illumination spot follows a separate focal surface during scanning and those focal surfaces are separated in a focus direction (i.e., axial direction) by a distance that exceeds a depth of focus of the optical system. Moreover, it may become difficult to simultaneously achieve acceptable optical aberration correction for a large number of beams at the final image in the multi-beam LSU when the beams are spread relatively far apart in the scan direction as a result of the tilted linear pattern.
Certain examples have other features that are one of in addition to and in lieu of the features illustrated in the above-referenced figures. These and other features are detailed below with reference to the preceding drawings.